1. Field of the Invention
The present invention relates to a magnetic recording medium and particularly to a so-called thermally-assisted magnetic recording medium where information is recorded by applying an external magnetic field while local heating is performed to a portion of the medium. More specifically, the present invention relates to the thermally-assisted magnetic recording medium where information is recorded by applying the external magnetic field while the local heating is performed to reduce coercive force of a recording film by laser light irradiation, near-field light irradiation and the like, and also relates to a magnetic recording/reproducing device using the thermally-assisted magnetic recording medium.
2. Description of the Prior Art
In recent years, regarding magnetic recording/reproducing devices such as magnetic disk devices, etc., it has been required to improve the respective performances of a thin film magnetic head and a magnetic recording medium in conjunction with higher recording density.
A magnetic recording layer of the magnetic recording medium mounted in a magnetic disk device is formed with an aggregation of magnetic microparticles, and each of the magnetic microparticles has a single magnetic domain structure. Then, one set of information in a perpendicular direction (one recording bit), for example, is recorded using a plurality of the magnetic microparticles. In order to enhance the recording density with the magnetic recording medium structured as described above, unevenness between the magnetic microparticles at boundaries of the recording bits needs to be reduced. Therefore, the volume V of the magnetic microparticles needs to be reduced.
However, thermal stability may be deteriorated when the volume V of the magnetic microparticles is excessively reduced, and therefore, magnetization directions may be disarranged because of the influence of temperature or the like. Accordingly, it is required to reduce the volume V of the magnetic microparticles without losing the thermal stability.
A thermal fluctuation index, which is an indication of the thermal stability, is given by KuV/KBT. Herein, Ku is an anisotropy energy constant of the magnetic microparticles, V is the volume of each magnetic microparticle, KB is Boltzmann constant, and T is the absolute temperature. When the volume of each magnetic microparticle is simply reduced to enhance the recording density, the thermal fluctuation index is lowered, the thermal stability is deteriorated, and therefore it becomes unable to perform the recording.
In order to solve such thermal fluctuation problem, it needs to use a magnetic material having a large anisotropy energy constant Ku. However, since the coercive force He of the magnetic microparticles becomes larger in proportion to the anisotropy energy constant Ku, a disadvantage occurs that it becomes unable to perform the recording with a conventional head.
As a method to solve such problem, a so-called thermally-assisted magnetic recording (TAMR) method has been proposed. The TAMR method is a method for recording information while He of the magnetic microparticles is reduced by heating a minute area (recording bit), which is a recording target of the magnetic recording medium, by laser light, near-field light or the like during the recording. Therefore, a magnetic material having a larger He than a recording head magnetic field H at a room temperature can be used. Also, the volume V of each magnetic microparticle can be reduced without lowering the thermal fluctuation index which is the indication of the thermal stability, and therefore it becomes possible to achieve the higher recording density.
JP Laid-Open Patent Application No. 2010-176747 and JP Laid-Open Patent Application No. 2009-158053, for example, disclose magnetic recording mediums which are preferable to be used for the conventional TAMR method.
JP Laid-Open Patent Application No. 2010-176747 discloses the magnetic recording medium which is used for a magnetic recording/reproducing device that performs a signal writing at a temperature higher than a temperature for a signal holding, and is configured by laminating a soft under layer, a thermal transfer prevention layer, an under layer, a magnetic recording layer, and a protective layer in this order. The magnetic recording medium of the publication features a thermal transfer prevention layer, which is formed of SiO2, TiO2, Y2O3, Al2O3, SiN, TiN, AlN, TiC, SiC or the like, that is disposed for the purpose of shielding thermal transfer from the recording layer side to the soft under layer. Similarly, the publication discloses that, for example, Ru, Re, Rh, Pt, Pd, Ir, Ni, Co, Cu or an alloy containing these materials is preferably used for the under layer, which controls a crystal lattice diameter and a crystal lattice orientation of the material of the recording layer and prevents a magnetic coupling between the soft under layer and the recording layer. Also, the publication discloses that a protective layer formed of carbon as a primary component is used for the protective layer formed on a magnetic layer, and that a lamination film configured with a metal film and a carbon film, and a lamination film configured with a film formed of a metal oxide and a carbon film can be used for the protective layer as well.
Also, JP Laid-Open Patent Application No. 2009-158053 discloses the magnetic recording medium that can be heated with less power consumption and also has an excellent writing performance, in which a cap layer formed of Fe—Pt alloy or Co—Cr—Cr—Pt—B alloy is disposed on a granular magnetic recording layer of the medium and a crystal orientation control and low thermal transfer intermediate layer formed of Ti—Al—V is disposed below the granular magnetic recording layer.
However, the media disclosed in the respective above-described publications are not media having a most preferable lamination film configuration as a magnetic recording medium used for thermally-assisted recording. With the mediums, spreading of a heating spot applied from a magnetic head for thermally-assisted recording to a film surface of the magnetic recording medium cannot be sufficiently suppressed so that an obstacle to achieve higher density recording may occur, and the magnetization direction of a perpendicularly written magnetization cannot be stabilized so that signal to noise ratio (SN) is deteriorated causing difficulties to achieve the higher recording density.
It is objective of the present invention, which is invented under such situations, to provide a thermally-assisted magnetic recording medium that can sufficiently suppress the spreading of the heating spot applied from the magnetic head for thermally-assisted recording to the film surface of the magnetic recording medium, that improves the SN by arranging the magnetization direction of the perpendicularly written recording magnetization to become identical to a perpendicular direction, and that can realize the higher recording density.